Gangliosides appear to be important recognition molecules on the cell surface and have been implicated as receptors for certain bacterial toxins and viruses. Little is know, however, about the normal physiological role(s) of these plasma membrane components. To explore the role of surface gangliosides in cell growth, we used the B or binding subunit of Cholera toxin as a specific probe for surface GM1. The only known function of the B subunit, which is multivalent, is to bind to the oligosaccharide chain of GM1. Exposure of quiescent rat thymocytes and mouse 3T3 cells to the B subunit resulted in a proliferative response as measured by increased DNA synthesis and cell numbers. The B subunit potentiated the effects of other mitogens such as epidermal growth factor, insulin and serum. As mitogens mediate their effects through several transmembrane signalling systems and induce the early expression of the proto-oncogenes c-myc and c-fos, the ability of the B subunit to initiate these early mitogenic responses was explored. The only acute response observed in both thymocytes and 3T3 cells exposed to the B subunit, was a rapid rise in intracellular free Ca2+ which depended on the presence of extracellular Ca2+. There was no elevation of cAMP, inositol phosphates, or intracellular pH, or activation of adenylate cyclase or protein kinase C. The B subunit also induced expression of c-myc and c-fos. Although the mechanism by which the B subunit triggers a rapid increase in intracellular free Ca2+ and induction of proto-oncogene expression is not yet known, it must be through its specific binding to GM1 on the cell surface. Thus, gangliosides may play a role as membrane transducers of signals that regulate cell growth.